1. Field of the Invention
The invention relates to a device for controlling the output power of laser diodes likely to be coupled to an optical fiber for the transmission of digital signals on optical fibers.
Laser diodes are widely used in the field of digital transmissions on optical fibers, but have the drawback of showing major variations of the luminous power emitted in response to a change in temperature or an ageing of the diode.
The diagrams of FIGS. 3 and 4 can be referred to for a clearer understanding of these phenomena.
FIG. 3 shows the variations of the characteristic curve of a laser as a function of the temperature. For a temperature T=20.degree. C., the curve C1 is obtained and for a temperature T=60.degree. C., the curve C2 is obtained. It is thus observed that, for a constant supply current I, the optical power emitted when the temperature is equal to 20.degree. C. is greater than the optical power emitted when the temperature is equal to 60.degree. C.
FIG. 4 shows two characteristic curves of laser diodes, one of which corresponds to a new laser diode and the other to a laser diode at the end of its lifetime. These characteristic curves have been plotted for a constant temperature equal to .THETA. .degree.C.
It can also be seen in this FIG. 4 that the optical power emitted is weaker for a given current I when the laser is at the end of its lifetime.
2. Background of the Invention
There is a regulation method used to compensate for the variations in the output power emitted by the diode laser that are due to a variation of the temperature. This method consists in carrying out an analog regulation of the luminous power emitted by a laser diode by comparing a signal representing said power with a reference signal to obtain an error signal that is used to modify the electrical supply current of the laser diode.
It has been observed that this method has the drawback of very low efficiency when the sending out of the data is periodic. Indeed, the loop which achieves a continuous regulation is found wanting for, as a representative signal, it uses a signal representing the sporadic power emitted.
Another known method consists solely of a temperature regulation. The regulation is done by cooling by means of a Peltier effect element controlled by a negative feedback loop, the input signal of which is constituted by the difference between the measured temperature of the laser and an instructed value of temperature. This servo-control system has the drawback of being costly and of consuming a great deal of energy, and of not taking account of the drift due to the ageing of the laser.
There also exist systems that foresee the end of the lifetime of the laser diode. These systems have a regulation arrangement to overcome the effects of variation due to the ageing of the diode, and also an active element to keep the operating temperature constant. Thus, the maximum current I indicating the end of the lifetime of the laser takes a fixed value, depending on the operating temperature which is kept constant. The diagnosis then amounts to a simple comparison of the value of the supply current of the laser with the value of current Imax that was fixed beforehand. A rudimentary system such as this remains costly and consumes a great deal of energy.
Apart from these drawbacks, the present applicant has also observed that there is a loss of optical power due to the coupling of the laser diode with the optical fiber, and that there is no system, at present, that enables total mastery to be achieved over this phenomenon. The applicant has also observed that the element which enabled the image of the information to be retrieved, generally a photodiode on the rear face of the laser, is generally disturbed by a change in temperature and that, consequently, the signal representing the luminous power emitted, is also subjected to variations due the variations in temperature. No existing regulation system enables this phenomenon to be taken into account.